Subsurface boring is increasingly important because it allows rapid placement of gas and water lines, sewers, electrical service, cable television service, and similar utility connections with minimal disturbance of roads, landscaping, buildings, and other surface features. Subsurface boring allows placement of pipe and utility connections where conventional surface installation by trenching is impractical or impossible as, for example, when utility connections must cross rivers, canals, major highways, or rail lines. Significant practical and economic advantages are derived from the ability to provide pipe and utility connections with minimal surface disturbance or to provide subsurface crossing of surface barriers.
However, realization of these advantages is dependent on the ability to properly locate and control the boring tool bit during operation. Accurate location of the boring tool is an especially important problem. In many boring tool systems, a small transmitting device called a beacon is installed immediately behind the drill bit. The electromagnetic field generated by the subsurface beacon is detected at the surface using devices variously called trackers or locators. The beacon's electromagnetic field is often modulated to convey subsurface information from the beacon to the system operator. Beacon signal amplitude, measured by one or a plurality of surface antennas, can be used to calculate distance between the underground beacon and the tracker.
Those familiar with the art recognize that the interaction of the antenna and the beacon's electromagnetic field can produce false, or "ghost", locations as indicated by the tracker. Locating on a ghost signal can be inconvenient and even dangerous as it presents a false picture of the beacon's location to the boring machine operator.